In recent years, in the fields in which metal materials were commonly used in the past, fiber-reinforced resin composite materials (hereinafter these will be referred to as “composite materials”) have been widely used. For example, a carbon fiber reinforced material (in general, referred to as carbon fiber reinforced plastics (CFRP)) formed by impregnating carbon fibers which are reinforced fiber material with a matrix resin including an epoxy resin or the like has a lighter weight and a higher strength than the metal materials do. For this reason, in recent years, the carbon fiber reinforced material has been widely used in the fields of sporting goods, industrial machines, vehicles (automobile, bicycle, etc.), aerospace, etc.
In general, a molded product comprising the composite material is obtained by laminating (stacking) plural sheets of prepreg together (sheets comprising a fiber reinforced material, impregnated with a matrix resin), and the resulting laminate is pressurized and heated to be cured in an autoclave.
Depending on a pressure applied to cure the uncured laminate or a temperature at which the laminate is heated, many minute vacancy defects called “porosities” emerge in a dispersed manner inside the composite material. If the porosities are present in a specified amount or more inside the composite material, this may lead to reduction of the structural strength of the composite material, in particular, in a case where the composite material is used in the field of aerospace, the structural strength requirements of the composite material is high, compared to a case where the composite material is used in other fields. Therefore, it is necessary to reduce the amount of porosities to a possible level. In view of this, when the composite material is manufactured, it is necessary to evaluate the porosities in a non-destructive manner.
Conventionally, as a technique for evaluating the defects of the composite material in the non-destructive manner, for example, there are an ultrasonic flaw detection method disclosed in Patent Literature 1, or an ultrasonic flaw detection device disclosed in Patent Literature 2. These techniques are intended for the composite material primarily used in the field of aerospace. In these techniques, an ultrasonic wave is input to the obverse surface of the composite material in a thickness direction, a reflective wave reflected on the reverse surface of the composite material and a transmissive wave measured on the reverse surface are measured, and the defects of the composite material are tested based on a degree to which the reflective wave or the transmissive wave is damped or the damping characteristics of the reflective wave or the transmissive wave.